The present invention relates to a sheet folding device that folds sheets carried out from an image forming apparatus such as a printer and a post-processing apparatus comprising the sheet folding device. More particularly, the present invention relates to improvements in a folding mechanism that folds a bunch of sheets together using a pair of folding rolls.
A device folding one or more overlapping sheets (bunch) together at a predetermined folding line is commonly known as a sheet folding device. Such a folding device is not only used as a single-function (standalone) device but also incorporated into, for example, a post-processing apparatus for an image forming apparatus as a sheet folding unit. For example, the sheet folding device is used to perform bookbinding by setting and stapling sheets with images formed thereon by an image forming apparatus such as a copier and folding the resultant sheet bunch at a half or one-third position into a booklet.
Among sheet folding mechanisms, a mechanism commonly adopted owing to the relatively simple structure thereof is the mechanism which folds the sheets by inserting the sheets between a pair of rolls that are in pressure contact with each other so that a fold position on the sheets first reaches the rolls.
For example, Patent Document 1 [Japanese Patent Laid-Open No. 2001-2317] discloses a device that sets sheets, from an image forming apparatus, into a bunch and then staples the sheet bunch at a central part thereof using staple means located on a sheet setting tray. Further, Patent document 1 discloses inserting the saddle-stitched sheet bunch between the paired rolls using a folding blade (folding plate) and folding the sheet bunch at a fold line position when the sheet bunch is delivered by the rolls. Further, Patent document 1 also states that the conventional art fails to take into account the specific relationship between the approach speed of the folding blade and the rotating peripheral speed of the rolls, resulting in breakage during sheet folding and thus discloses that a reduced rotating peripheral speed is set for the folding rolls relative to the speed of the folding blade in order to solve this problem.
Further, according to Patent document 1, a mechanism is disclosed in which sheet holding means for holding a sheet bunch has a pair of folding rolls arranged at a central fold line position thereof. Further, the Patent document 1 also discloses a folding blade which (folding plate) angles and inserts the sheet bunch between roll nips and the rolls then folding the sheet bunch. The folding rolls are wider than the sheets and are constructed to have a roll shape (cylindrical shape) so as to evenly urge the entire sheet bunch in a sheet width direction. However, when the cylindrical rolls that are in pressure contact with each other, fold the sheets (one or more sheets) together, the sheets are more likely to be rucked which is a disadvantage.
Further, Patent Document 2 [Japanese Patent Laid-Open No. 2000-327209] describes that when a sheet bunch is stapled and folded at a fold line position, a ruck may be developed which extends rearward from a staple end surface (fold line edge). That is, when sandwiched between the cylindrical folding rolls, the sheet bunch stapled at the fold line may be corrugated at a trailing end thereof, with a ruck being developed. Patent Document 2 thus proposes that the folding rolls be partly cut into crescentic shapes so as to heavily urge the fold line portion by the peripheral surfaces thereof, while avoiding sandwiching an area behind the fold line between the folding rolls. Further, Patent Document 2 also discloses a cam structure that releases the pressure contact of the folding rolls.
Patent Document 3 [Japanese Patent Laid-Open No. 10-167562] discloses a folding mechanism having folding rolls, which are in pressure contact with each other and which are arranged above sheets, The sheets are carried out from an image forming apparatus and set and collected into a bunch. The folding mechanism also includes a folding blade feeding the sheet bunch from an opposite side of the collecting section to between roll nips. The folding rolls are arranged so as to substantially contact a surface of the sheet bunch collected in the collecting section. Simultaneously with the driving rotation of the pair of rolls, the folding blade bends the sheet bunch at the fold position and inserts the sheet bunch between the roll nips. Patent Document 3, further discloses sheet guides larger than the diameter of the rolls provided around the outer peripheries of the rolls to prevent the sheet bunch from being subjected to a ruck or the like when the sheet bunch is delivered to the nip position.
Similarly, Patent Document 4 [Japanese Patent Laid-Open No. 2002-145516] discloses a folding mechanism having folding rolls offset from sheets collected into a bunch, by a predetermined distance, and a folding blade that feeds the sheet bunch to between roll nips. Thus, the conventional documents commonly disclose a mechanism which has a pair of folding rolls that are in pressure contact with each other to fold a sheet bunch into a booklet and in which the sheet bunch is fed so as to be folded at a fold line position by the folding blade and is nipped and folded between the folding rolls.
Patent Document 5 [Japanese Patent Laid-Open No. 2001-302089] proposes a mechanism that balances a load acting on a leading end of sheets with a load acting on a trailing end of the sheets in order to prevent the sheets from being misaligned when a folding blade inserts the sheets between the nips of folding rolls. The misalignment may occur when the collected sheets are curled or when the loads of a conveying roller and the like act biasedly on the sheets.
In any of the above-described conventional techniques, the sheet guide, which holds the sheets at the fold position, collects the sheets on a substantially flat plane. The folded sheet bunch is discharged straight to the exterior of the device in a direction in which the sheet bunch is inserted by the blade.
The following problems and disadvantages are involved in sheet folding devices using the above-described conventional techniques and post-processing apparatuses and image forming systems comprising the sheet folding devices.
First, when the folding blade inserts the bunched sheets to between the rolls that are in pressure contact with each other, the following problem occurs. As pointed out in Patent Document 1, when the peripheral speed of the roll pair is higher than the moving speed of the folding blade, an outer sheet contacting the roll pair is fed between the paired rolls faster than an inner sheet contacting the folding blade. Consequently, while the sheet bunch is being stapled, a trouble such as a ruck or damage may occur on the outer sheet. Furthermore, even if the roll speed is the same as the blade speed, the pressure contact force between the rolls, the frictional force among the sheets, and the like may vary. Further, an external shock may make the conveying force of the roll pair and the conveying force of the folding blade unequal, both of which are exerted on the sheets. This may result in a ruck or damage as described above.
Furthermore, when the moving speed of the folding blade is set higher than the peripheral speed of the rolls as described in Patent Document 1, a difference in speed may occur between the roll pair and the sheet bunch inserted between the rolls by the folding blade, particularly between the roll pair and the outer sheet. Therefore, a slip between the roll pair and the sheet surface may occur. The slip may cause the applied image ink to be rubbed, leading to a blurred image. Patent document 1 fails to prevent the image rubbing which adds to the disadvantage. In particular, if for example, an image is formed on a cover sheet positioned outside the sheet bunch, the problem significantly affects finish quality.
The above-mentioned problem associated with sheet folding frequently occurs when the folding roll diameter is reduced to miniaturize the device. It is also known that the increased number of sheets in the sheet bunch increases the frequency with which a folding defect such as a ruck or damage occurs. For example, when a sheet bunch, with a large number of sheets, is fed to between the rolls while being folded along the fold line by the folding blade, the outer sheet (cover sheet or the like), first contacting the roll pair, is caught between the rolls before and separately from the other sheets. This causes the above-described problem.
Therefore, it is important to identify that the problem such as a ruck, damage, or image rubbing which is associated with folding of the sheet bunch is attributed to the difference in speed between the folding rolls and the folding blade. Thus, it is an object of the present invention to obviate the above-mentioned problems by inserting the bunched sheets to the nip position on the folding rolls at the moving speed of the folding blade, and after the sheets reach the nip position, feeding the sheets at the peripheral speed of the roll pair.
Second, when the sheets consecutively carried out from the image forming apparatus are set, collected, and folded into a booklet as described above, the sheets are collected on a flat surface, and the folding rolls are arranged in contact with the top surface of the sheets as described in Patent Document 3. Furthermore, in Patent Document 4, the sheets are supported at the fold position by the sheet guides, composed of substantially flat planes. In this condition, to insert the sheets between the roll nips, the folding blade projects the sheet bunch so as to bend the sheet bunch at the fold line.
Thus, with the conventional folding devices, the sheet bunch is supported on the guides, composed of substantially flat surfaces. The flat sheet bunch is thus inserted between the nips of the folding rolls while being bent by the folding blade. Thus, when the folding rolls are arranged in contact with and in proximity to the surface of the sheet bunch as described in Patent Document 1, the upper layer sheet is first delivered by the folding rolls when the folding blade projects the sheet bunch. This may result in a void between the stacked sheets. The presence of the void between the sheets at the fold position may make the leading ends (fore edges) of the sheets loose, degrading folding quality. Furthermore, a friction mark of the folding rolls may be left on the front layer sheet, for example, leaving an abrasion mark on an image forming surface.
Thus, in Patent Document 3, the folding rolls are arranged downstream of and offset from the sheet bunch fold position by a certain distance. The sheet bunch is bent into an angular form using the folding blade and the sheet guides, and nipped and folded between the folding rolls. However, the adoption of this mechanism poses a new problem described below.
In the conventional art, the folding rolls are arranged in contact with the sheet bunch placed flat, and the folding blade inserts the sheet bunch to the nip position along the outer peripheries of the rolls. In contrast, the folding rolls offset from the sheet bunch by a certain distance may disadvantageously result in a ruck on the sheets or misalignment of the fold line position when the sheet bunch is bent by the folding blade before being nipped between the rolls. That is, the sheet bunch placed flat may contain a sheet curled in a direction opposite to that in which the sheet bunch is to be bent. When pushed at the fold position by the folding blade, the sheet curved in the direction opposite to the sheet bunch bending direction and sandwiched between flat sheets may be rucked in the central part thereof or the fold position may deviate from the correct one. When the sheets set at the fold position include those curved in the inserting direction and those curved in the opposite direction, the sheets may be rucked or the fold position may be misaligned when the sheets are folded together.
The rucked sheets or the misalignment of the sheet leading ends after the folding may occur if a void is present between the collected sheets or if the direction in which the curled sheet is curled is opposite to the bending direction. A possible solution to this problem is to tightly support the sheet bunch so as to avoid creating a void between the sheets when the sheet bunch is held at the fold position and to prevent the curled sheet from being bent in the opposite direction when the folding blade bends the sheet bunch.
Third, with the sheet folding mechanism in which the folding blade inserts the sheets (sheet bunch) between the nips of the pair of folding rolls that are in pressure contact with each other so that the fold line position first reaches the nips as described above, the sheets may be rucked. In this rucking condition, when folded at the fold line position by the folding rolls, the sheets (sheet bunch) may be rucked at the trailing end thereof. To prevent this, the conventional art, for example, the structure in Patent Document 1, precisely sets the diameter and axial parallelism of the folding rolls. Thus, disadvantageously, this structure is difficult to manufacture and requires advanced techniques for maintenance during use. Furthermore, it appears that the structure in Patent Document 2, although, might prevent possible rucking resulting from stapling of the sheets at the fold line position, cannot prevent possible rucking occurring as described below.
When the sheets (sheet bunch) are sandwiched between and folded together by the pair of folding rolls and a mechanism is adopted which adjusts the sandwiching force of the rolls exerted on the fold line on the sheets and on an area located behind the fold line, a timing at which the rolls pressurizes the sheets and a timing at which the folding blade inserts the sheets may vary with the operation. If the timings deviate from each other and the sheet pressurizing timing is delayed, the sheets cannot be reliably folded together at the fold line. Furthermore, if a timing at which the sandwiching of the sheets is released is delayed, the sheets may be rucked in an area thereof behind the fold line.
In the structure as described in Patent Document 2, the folding rolls are shaped like crescents or composed of cams rotating around a driving shaft so that a part of the peripheral surface of each of the rolls pressurizes the sheets, while a different part of the peripheral surface avoids pressurizing the sheets. Thus, to fold longer sheets together, the rotation angle of the folding rolls must be accurately controlled in accordance with a sheet insertion condition (timing). On the other hand, when the sheets are inserted to the nip position by the folding blade, the timing of actuation of the folding blade and a timing at which the sheets reach the nip position vary with the operation depending on a load imposed on the sheets being bent and deformed at the fold line. For example, the timings are advanced when one thin sheet is folded and are delayed when a large number of thick sheets are folded.
It is thus difficult to control the rotation angle of the folding rolls and the operation of the folding blade in accordance with fixed timings regardless of the thickness of the sheet or the sheet bunch. Consequently, the sheets are unavoidably rucked in the area thereof behind the fold line. In particular, if the diameter of the folding rolls is reduced to miniaturize the device or sheets of various sizes or thicknesses are used, rucking occurs frequently as described above.
It is therefore necessary to release the pressurizing force in conjunction with the timing of movement of the folding blade inserting the sheets between the nips of the folding rolls, the sheets can be folded at the exact fold line position, with the pressurization of the area of the sheets behind the fold line avoided, regardless of the sheet size or thickness or the sheet bunch thickness.
Fourth, when the sheet bunch with the stacked sheets is sandwiched between and folded together by the paired rollers that are in pressure contact with each other, the sheet bunch nipped between the paired rollers is conventionally carried out in a linear direction orthogonal to the nip direction. This is because the folding rolls are normally shaped like rounds or cylinders longer than the width of the sheets, so that rucking or the like may occur unless the folded sheet bunch is carried out to a tray located outside the device, along a straight path orthogonal to the nip direction.
When the folded sheet bunch is carried out from the folding rolls to the exterior of the device along the straight, linear path orthogonal to the nip direction as described in, for example, Patent Document 1, spatial limitations are imposed on the layout of the device. For example, the device configuration disclosed in Patent Document 1 requires paper guides holding the sheets in front of and behind the folding rolls. Thus, to curve the sheets fed in a horizontal direction by the image forming apparatus, in a vertical direction to collect the sheets in upright bunch form and to fold and carry out the sheet bunch by the folding rolls, a sheet discharging tray must be located on the path orthogonal to the roll pair. This is because the sheet bunch folded into a booklet is not readily curved or deformed, so that the sheet discharging tray cannot be located away from the folding rolls, for example, above or below the folding rolls.
Thus, an attempt is also made to use sheet discharging rollers and guides to forcibly curve the sheet bunch from the folding rolls during a carry-out operation. However, the front layer sheet in the sheet bunch may be rucked. Furthermore, during the carry-out operation, the leading end of the sheet bunch is curved and deformed while rubbing against surfaces of the guides. This increases a conveying load for an elastic sheet bunch such as a bunch of cardboards, resulting in a jam or an increase in the size of the device or in manufacturing costs owing to the need for a driving motor of a higher capacity than required or the like.
The above-described problems can be solved by controlling the rotation of the pair of folding rolls so that the sheet bunch is curved, in the device configuration in which the folded sheet bunch is carried out to the sheet discharging tray through the sheet discharging guides curved upward or downward with respect to the direction in which the sheet bunch is inserted by the blade.
Fifth, to collect the sheets into a bunch at the fold position and to fold the sheet bunch by the folding rolls, a mechanism is conventionally used which brings an edge (leading or trailing end) of the sheets into abutment with a regulating member and which folds the sheets together using a knife-like blade (folding plate). If this folding mechanism is used for sheets of different lengths, the collected sheets need to be moved to a predetermined fold position. For example, the sheets may be misaligned when moved along the sheet guides in the order of the staple position and the fold position.
In particular, if the sheets are set and collected in a substantially horizontal posture or a substantially vertical upright posture, the sheets may be misaligned when the sheet bunch is moved to the predetermined staple or fold position according to the sheet size. The movement of the sheets is conventionally performed by the conveying roller for horizontal support or by a leading end regulating member for vertical support. In this case, with the roller, a variation in frictional force between the sheets may misalign the leading ends of the sheets. With the leading end regulating member, curled sheets may misalign the leading ends of the sheets.
It is therefore an object of the present invention to provide a sheet folding device that prevents a sheet bunch from being misaligned, rucked, or damaged when inserted and folded together between paired rolls, thus providing excellent finish quality.
A further object of the present invention is to provide a post-processing apparatus and an image forming system which can collect sheets sequentially carried out from an image forming apparatus, into a bunch and then fold the sheet bunch at a predetermined fold line.
The present invention achieves not only the above-described objects but also objects described below. The above-described problems are expected to be solved by tightly supporting a sheet bunch held at a fold position so as to avoid creating a void between the sheets, and when the folding blade bends the sheet bunch, preventing the curled sheets from being bent in the opposite direction. This is because the sheets may be rucked when a void is present between the sheets and because rucking or misalignment of the leading ends of the folded sheets may occur if a direction in which curled sheets are curled is opposite to a bending direction. One of the objects is thus to provide a sheet folding device that, when the sheets are folded, prevents the possible rucking and the possible misalignment of the leading edges of the folded sheets and an image forming system that, when curled sheets carried out from an image forming apparatus or the like are set, collected, and folded, prevents the possible rucking and the possible misalignment of the leading edges of the folded sheets.
Yet another object of the present invention is to provide an inexpensive sheet folding device of a simple structure which, when sheets are folded by a pair of folding rolls, prevents the sheets from being rucked and also prevents a friction mark of the folding rolls from being left on the sheets.
Another object is to provide a sheet folding device that allows the sheet bunch folded by the folding rolls to be smoothly carried out to a sheet discharging tray located above or below the folding rolls in the device without damaging the sheet bunch, thus providing a compact device layout using a reduced amount of space.
Another object is to provide a compact, inexpensive image forming system that sets and collects sheets carried out from the image forming apparatus, into a bunch and then folds the sheet bunch into a booklet.
Another object is to provide a sheet folding device which, when the sheets collected into a bunch are moved to the predetermined fold position for setting, prevents the possible misalignment of sheet edges and which, when the sheet bunch is folded together at the fold position, prevents the possible misalignment of a fold line.
Further objects and advantages of the invention will be apparent from the following description of the invention.